In the past, in manufacturing an ultra thin polymer film electret, a thin polymer film was formed by a melt extrusion method, a coating method, a physical deposition method and by applying an electric field directly to the thin polymer film together with light, heat or mechanical strength.
In the prior art, a thin polymer film having a thickness of about a dozen .mu.m was formed. However, the orientation at the molecular level was not high. For that character of the thin polymer film it was difficult to obtain an electret which had a sufficient surface electric charge even though high voltage was applied.
In the prior method of forming a thin polymer film, it was also difficult to control the thickness of film at the molecular level. Thus, if the thickness of film was only at the molecular level, it could not be applied uniformly thin. The result was that the film had pin-holes and an island-state structure.
Further, in order to control orientation of a polymer, a film was drawn. However, in that case, there was a limitation as to how thin a film could be drawn, and the orientation of molecules could not be controlled sufficiently. Thus, in the prior art, only an electret which had a film layer more than about several hundred to several thousand angstroms in thickness could be obtained. In addition, the orientation of molecules was not high. Thus, if the thickness of the film was decreased, the surface electric charge became small.
For the above reasons, the object of the invention is to provide an ultra thin polymer film electret which is much thinner than prior polymer electrets and also has sufficient surface electric charge and a method of making the same.